Device for clamping an ophthalmic lens

ABSTRACT

A device for clamping an ophthalmic lens for edging it includes a driving abutment including a central peg extended externally by an elastically deformable flange.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a device for clamping an ophthalmic lens, for manipulating the lens while edging it, for example by grinding it, to match its contour to that of the rim of the chosen frame. The invention relates more particularly to an improvement made to a driving abutment of the device. A preferential application of the invention is to edging an ophthalmic lens having a surface layer of very low surface energy to facilitate cleaning the lens.

[0003] 2. Description of the Prior Art

[0004] Edging an ophthalmic lens by grinding it necessitates clamping and holding the lens between two coaxial shafts whose ends are provided with driving abutments, each having a flexible material facing that is sufficiently thick to adapt to differences in the geometries of different lenses. The machining references of the lens are conventionally provided by an accessory adapted to constitute one of the aforementioned driving abutments, i.e. adapted to be attached to the end of one of the shafts. This elastomer material accessory is stuck to the front face of the ophthalmic lens at a location that depends in particular on the optical characteristics of the lens and on data representative of the morphology of the wearer. It is stuck by means of a double-sided adhesive. When the lens has been placed, by means of the accessory forming a driving abutment, at one end of one of the shafts, it is clamped by the driving abutment fitted to the end of the other shaft. The latter is generally mounted on a ball-joint. This solution has a number of drawbacks. On the one hand, the adhesion between the elastomer material and the lens is relatively low, and this problem tends to be accentuated by the widespread adoption of surface coatings to facilitate cleaning, which have a very low surface energy. On the other hand, the elastomer being relatively thick, the driving abutments tend to deform in torsion when machining forces are applied to the lens. This can result in loss of the reference position of the lens to be ground, the principal outcome of which is axis errors and sometimes centering errors. The invention overcomes these drawbacks.

SUMMARY OF THE INVENTION

[0005] To be more precise, the invention provides a device for clamping an ophthalmic lens for edging it, which device includes a driving abutment including a relatively deformable central peg extended externally by a semi-rigid elastically deformable and flexible material flange and at least said flange is conformed to have an application surface adapted to come into contact with said lens and to be deformed to espouse the shape thereof by flexing freely due to the effect of a clamping force applied to said abutment.

[0006] The device can include, or be used with, an accessory forming a driving abutment adapted to be fitted to the end of one of the shafts, as in the prior art. In this case, the other driving abutment, cooperating with the rear face of the lens, is of the type conforming to the invention defined hereinabove. It is also feasible for the removable accessory forming a driving abutment to conform to the invention. In this case, the application surface is provided with a double-sided self-adhesive coating.

[0007] The invention also applies to an ophthalmic lens clamping device with automatic positioning, and in this case the two shafts are each provided with a driving abutment conforming to the invention. When the two shafts are urged toward each other axially, each driving abutment is deformed to espouse the shape of the lens.

[0008] The initial shape of the flange (i.e. its shape in the unstressed state) is different from that of the surface of the lens to which it must be applied. Due to the effect of the axial force, the flange flexes freely because there is no rigid member forming an abutment behind it. This elastic flexing (causing the deformation of the flange) contributes to proper distribution of the clamping force.

[0009] As will emerge later, the thickness of the flange is calculated to obtain a relatively constant pressure over the whole of the contact area. If the driving abutment is intended to cooperate with the rear face of an ophthalmic lens, it is advantageous for it to have a substantially plane application surface in the unstressed state. If the driving abutment is intended to cooperate with the front face of an ophthalmic lens, it is advantageous for it to have a concave surface in the unstressed state.

[0010] According to another advantageous feature of the invention, the application surface can be covered with a thin plastics material or elastomer material facing, and the thickness of the facing is preferably of the order of 1 to 2 mm. Also, this material preserves the machining references in all circumstances. For example, the thin coating can consist of flexible PVC or neoprene.

[0011] When the driving abutment constitutes a positioning accessory that is placed on the front face of the lens, the application surface is provided with an adhesive layer such as a double-sided self-adhesive coating. This adhesive material can replace the thin plastics material or elastomer material facing.

[0012] The flange of the driving abutment will generally have a circular contour. However, for grinding a narrow lens (a half-moon lens, for example), the flange of this kind of driving abutment can have an oblong contour. The flange and the peg are preferably made in one piece and from the same material. Satisfactory results have been obtained using polyoxymethylene. In one embodiment using polyoxymethylene, the flange had a thickness of the order of 1.5 mm.

[0013] The invention will be better understood and other advantages of the invention will become more clearly apparent in the light of the following description, which is given by way of example only and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagrammatic view of part of a device in accordance with the invention for clamping an ophthalmic lens, showing the shapes of driving abutments on respective opposite sides of the lens before clamping.

[0015]FIG. 2 is a view analogous to FIG. 1 showing the shapes of the driving abutments after clamping.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The clamping device 11 shown in part includes two axially aligned shafts 12, 14 adapted to be moved toward each other. Each shaft 12, 14 carries at its end a respective driving abutment 16 a, 16 b. An ophthalmic lens 20 can be clamped between the two abutments. In the conventional way, the clamping device 11 is adapted to move in front of a grinding wheel (not shown). This is known in the art. Also, after clamping, the two shafts 12, 14 are driven in rotation to turn the ophthalmic lens 20 relative to the grinding wheel. The required contour is imparted to the ophthalmic lens by controlling the rotation of the lens and the distance between the common axis of the two shafts and the rotation axis of the grinding wheel. In the example shown, the clamping device 11 is of the automatic positioning type, i.e. the ophthalmic lens is carried by a support, not shown, and is positioned relative to the common axis of the two shafts so that the center of the clamping area coincides with a predetermined point on the ophthalmic lens that depends in particular on specific characteristics of the lens and characteristics representative of the morphology of the wearer. Conventionally, the clamping point also corresponds to the center of the rectangle in which is inscribed the rim of the frame, also called the surround.

[0017] As shown here, each driving abutment 16 a, 16 b includes a relatively undeformable central peg 24 a, 24 b extended externally by an elastically deformable flange 26 a, 26 b. Each flange is made of a semi-rigid plastics material and consequently is able to flex elastically and freely (being unrestrained) when it is pressed onto the lens. The flange is conformed with an application surface 28 a, 28 b adapted to come into contact with the lens and to espouse its shape due to the effect of a clamping force applied to the corresponding abutment when the two shafts are moved toward each other. The configuration after clamping is that shown in FIG. 2. In this example, the peripheral part of the application surface is part of the flange and its more central part is part of the peg itself. However, there is no break in continuity between the part of the application surface 28 a, 28 b that is part of the flange 26 a, 26 b and that which is part of the peg 24 a, 24 b. Moreover, in this example, the application surface of each driving abutment is covered with a thin plastics material or elastomer material facing 30. The thickness of the facing is of the order of 1 to 2 mm. It consists of flexible PVC or neoprene, for example.

[0018] As can be seen in FIG. 1, the two driving abutments 16 a, 16 b are not exactly the same shape. The abutment 16 a that is intended to cooperate with the front face of the ophthalmic lens has a concave application surface 28 a in the unstressed state. The abutment 16 b that is intended to cooperate with the rear face of the ophthalmic lens has a substantially plane application surface 28 b in the unstressed state. In the case of a device with manual positioning, in which the driving abutment attached to the shaft 12 is in fact a removable accessory that can be stuck to the front surface of the ophthalmic lens 20 before mounting and clamping the lens, the thin plastics material or elastomer material facing 30 can be replaced by a double-sided adhesive, because the adhesive will have a behavior comparable to that of the thin plastics material or elastomer material facing of the other driving abutment 16 b during edging of the lens, in other words it will effectively oppose any deformation in torsion and will consequently prevent any loss of the positioning reference.

[0019] The central part of the application surface 28 a, 28 b of each driving abutment 16 a, 16 b is recessed, the central peg featuring an axial bore. Moreover, the peg and the flange are made in one piece from the same material. Satisfactory results have been obtained by making the abutment from polyoxymethylene having a Young's modulus E=2 900 N/mm². Good results can be obtained by choosing a thickness h of the flange equal to 1.5 mm.

[0020] More generally, the approximate thickness of the flange is given, as a function of the clamping force F, the desired deflection w (in mm) of the outside of the flange when a force F is applied, and the Young's modulus E (in N/mm²) of the chosen material, by the equation: ${h = {\sqrt{\frac{12\left( {1 - v} \right)F\quad a^{3}}{2\pi \quad b\quad E\quad w}}\left( {\frac{\alpha\beta}{\gamma}ɛ} \right)}},$

[0021] in which a is the outside radius of the flange, b is the inside radius of the flange (in mm) and v is Poisson's coefficient for the material (usually 0.3).

[0022] The parameters α, β, γ and ε are defined by the following equations: $\alpha = {\frac{1}{4}\left\lbrack {1 - {\left( \frac{b}{a} \right)^{2}\left( {1 + {2\quad {\ln \left( \frac{a}{b} \right)}}} \right)}} \right\rbrack}$ $\beta = {\frac{b}{a}\left\lbrack {{\frac{1 + v}{2}{\ln \left( \frac{a}{b} \right)}} + {\frac{1 - v}{4}\left( {1 - \left( \frac{b}{a} \right)^{2}} \right)}} \right\rbrack}$ $\gamma = {\frac{1}{2}\left\lbrack {1 + v + {\left( {1 - v} \right)\left( \frac{h}{a} \right)^{2}}} \right\rbrack}$ $ɛ = {\frac{b}{4a}\left\lbrack {{\left( {\left( \frac{b}{a} \right)^{2} + 1} \right){\ln \left( \frac{a}{b} \right)}} + \left( \frac{b}{a} \right)^{2} - 1} \right\rbrack}$

[0023] A plastics material or an elastomer having the highest possible adhesion to the lens is used for the thin facing for improving adhesion. The two materials indicated hereinabove are satisfactory. The Shore hardness of the material is preferably less than 90. 

1. A device for clamping an ophthalmic lens for edging it, which device includes a driving abutment including a relatively deformable central peg extended externally by a semi-rigid elastically deformable material flange and at least said flange is conformed to have an application surface adapted to come into contact with said lens and to be deformed to espouse the shape thereof by flexing freely due to the effect of a clamping force applied to said abutment.
 2. The device claimed in claim 1, wherein a peripheral part of said application surface is part of said flange and its more central part is part of said peg.
 3. The device claimed in claim 1, wherein said application surface is covered with a thin plastics material or elastomer material facing.
 4. The device claimed in claim 3, wherein the thickness of said thin facing is of the order of 1 to 2 mm.
 5. The device claimed in claim 3, wherein said thin facing is made of flexible PVC.
 6. The device claimed in claim 3, wherein said thin facing is made of neoprene.
 7. The device claimed in claim 1, wherein said application surface includes an adhesive layer.
 8. The device claimed in claim 1, wherein a driving abutment adapted to cooperate with the rear face of an ophthalmic lens has a substantially plane surface in the unstressed state.
 9. The device claimed in claim 1, wherein a driving abutment adapted to cooperate with the front face of an ophthalmic lens has a concave surface in the unstressed state.
 10. The device claimed in claim 1, wherein said driving abutment flange has an oblong contour.
 11. The device claimed in claim 1, wherein said flange and said peg are made in one piece from the same material.
 12. The device claimed in claim 11, wherein said material is polyoxymethylene.
 13. The device claimed in claim 12, wherein the thickness of said flange is of the order of 1.5 mm. 